Genome-wide scan for linkage to schizophrenia in a Spanish-origin cohort from Costa Rica

Genetic studies of neuropsychiatric disorders in Costa Rica: a model for the use of isolated populations

The importance of genetics in understanding the etiology of mental illness has become increasingly clear in recent years, as more evidence has mounted that almost all neuropsychiatric disorders have a genetic component. It has also become clear, however, that these disorders are etiologically complex, and multiple genetic and environmental factors contribute to their makeup. So far, traditional linkage mapping studies have not definitively identified specific disease genes for neuropsychiatric disorders, although some potential candidates have been identified via these methods (e.g. the dysbindin gene in schizophrenia; Straub et al., 2002; Schwab et al., 2003). For this reason, alternative approaches are being attempted, including studies in genetically isolated populations. Because isolated populations have a high degree of genetic homogeneity, their use may simplify the process of identifying disease genes in disorders where multiple genes may play a role. Several areas of Latin America contain genetically isolated populations that are well suited for the study of neuropsychiatric disorders. Genetic studies of several major psychiatric illnesses, including bipolar disorder, major depression, schizophrenia, Tourette Syndrome, alcohol dependence, attention deficit hyperactivity disorder, and obsessive-compulsive disorder, are currently underway in these regions. In this paper we highlight the studies currently being conducted by our groups in the Central Valley of Costa Rica to illustrate the potential advantages of this population for genetic studies.

ProtocadherinX/Y, a candidate gene-pair for schizophrenia and schizoaffective disorder: a DHPLC investigation of genomic sequence

Protocadherin X and Protocadherin Y (PCDHX and PCDHY) are cell-surface adhesion molecules expressed predominantly in the brain. The PCDHX/Y gene-pair was generated by an X-Y translocation approximately 3 million years ago (MYA) that gave rise to the Homo sapiens-specific region of Xq21.3 and Yp11.2 homology. Genes within this region are expected to code for sexually dimorphic human characteristics, including, for example, cerebral asymmetry a dimension of variation that has been suggested is relevant to psychosis. We examined differences in patients with schizophrenic or schizoaffective psychosis in the genomic sequence of PCDHX and PCDHY in coding and adjacent intronic sequences using denaturing high performance liquid chromatography (DHPLC). Three coding variants were detected in PCDHX and two in PCDHY. However, neither the coding variants nor the intronic polymorphisms could be related to psychosis within families. Low sequence variation suggests selective pressure against sequence change in modern humans in contrast to the structural chromosomal and sequence changes including fixed X-Y differences that occurred in this region earlier in hominid evolution. Our findings exclude sequence variation in PCDHX/Y as relevant to the aetiology of psychosis. However, we note the unusual status of this region with respect to X-inactivation. Further investigation of the epigenetic control of PCDHX/Y in relation to psychosis is warranted.

NR4A2 and schizophrenia: lack of association in a Portuguese/Brazilian study

The present study investigates the association of mutations in the nuclear receptor NR4A2 in schizophrenic patients. The human Nur-related receptor 1, NR4A2, is an orphan nuclear receptor that can be constitutively active as a transcription factor and for which no natural ligand has yet been identified. Alone or with retinoid X receptor, RXR, NR4A2 influences the expression of several genes important for human brain development and regulation. In the absence of Nurr1 (the mouse homologue to human NR4A2), ventral mesencephalic dopaminergic mouse neurons evidence severe developmental failure, a condition that is lethal soon after birth. Nurr1 involvement in the dopaminergic system makes it a good candidate for study in neuropsychiatric disorders such as schizophrenia and Parkinson disease. Evidence by others support this hypothesis (1) mapping of the NR4A2 gene to chromosome 2q22-23, a region with suggestive linkage to schizophrenia and (2) identification of mutations in patients with schizophrenia (c.366-369delTAC, c.308A > G, c.-469delG), manic depression (c.289A > G), and familial Parkinson's disease (c.-291delT, c.-245T > G). To further extend these observations, we searched for all these mutations in 176 Caucasian Portuguese and 82 Caucasian Brazilian subjects with lifetime diagnosis of schizophrenia. The study failed to identify any of the described mutations in patients or controls. Nevertheless, these negative results do not exclude altered expression of nuclear receptors in schizophrenia or the presence of other mutations.

Expression analysis of neuregulin-1 in the dorsolateral prefrontal cortex in schizophrenia

Genetic linkage and association have implicated neuregulin-1 (NRG-1) as a schizophrenia susceptibility gene. We measured mRNA expression levels of the three major isoforms of NRG-1 (ie type I, type II, and type III) in the postmortem dorsolateral prefrontal cortex (DLPFC) from matched patients and controls using real-time quantitative RT-PCR. Expression levels of three internal controls-GAPDH, cyclophilin, and beta-actin-were unchanged in schizophrenia, and there were no changes in the absolute levels of the NRG-1 isoforms. However, type I expression normalized by GAPDH levels was significantly increased in schizophrenia DLPFC (by 23%) and positively correlated with antipsychotic medication dosage. Type II/type I and type II/type III ratios were significantly decreased (18 and 23% respectively). There was no effect on the NRG-1 mRNA levels of genotype at two SNPs previously associated with schizophrenia, suggesting that these alleles are not functionally responsible for abnormal NRG-1 expression patterns in patients. Subtle abnormalities in the expression patterns of NRG-1 mRNA isoforms in DLPFC may be associated with schizophrenia.

Genomewide scan in families with schizophrenia from the founder population of Afrikaners reveals evidence for linkage and uniparental disomy on chromosome 1

We report on our initial genetic linkage studies of schizophrenia in the genetically isolated population of the Afrikaners from South Africa. A 10-cM genomewide scan was performed on 143 small families, 34 of which were informative for linkage. Using both nonparametric and parametric linkage analyses, we obtained evidence for a small number of disease loci on chromosomes 1, 9, and 13. These results suggest that few genes of substantial effect exist for schizophrenia in the Afrikaner population, consistent with our previous genealogical tracing studies. The locus on chromosome 1 reached genomewide significance levels (nonparametric LOD score of 3.30 at marker D1S1612, corresponding to an empirical P value of.012) and represents a novel susceptibility locus for schizophrenia. In addition to providing evidence for linkage for chromosome 1, we also identified a proband with a uniparental disomy (UPD) of the entire chromosome 1. This is the first time a UPD has been described in a patient with schizophrenia, lending further support to involvement of chromosome 1 in schizophrenia susceptibility in the Afrikaners.

Genetic mapping using haplotype and model-free linkage analysis supports previous evidence for a locus predisposing to severe bipolar disorder at 5q31-33

We report further evidence for our previous suggestion [Garner et al., 2001: Am J Hum Genet 68:1061-1064] of a locus on 5q predisposing to bipolar I disorder (BP-I) in an extended Costa Rican pedigree. We genotyped additional microsatellite markers in this region and applied a multi-point non-parametric linkage analysis (SimWalk2). Significant identity-by-descent allele sharing among affected relatives was observed for all of the 20 markers tested in a segment of approximately 15 cM. Most affected individuals shared a single haplotype over this region; breaks within this haplotype may suggest a more restricted candidate location for a BP-I gene. These results support the suggestion that a locus at 5q31-33, together with a previously reported locus at 18q22-23, may provide the major genetic risk for BP-I in this family.

Genome-wide scan in Portuguese Island families identifies 5q31-5q35 as a susceptibility locus for schizophrenia and psychosis

Schizophrenia is a common psychiatric disorder with a complex genetic etiology. To understand the genetic basis of this syndrome in Portuguese Island populations, we performed a genome-wide scan of 29 families with schizophrenia, which identified a single region on 5q31-5q35 with strong linkage (NPL=3.09, P=0.0012 at D5S820). Empirical simulations set a genome-wide threshold of NPL=3.10 for significant linkage. Additional support for this locus in schizophrenia comes from higher-density mapping and mapping of 11 additional families. The combined set of 40 families had a peak NPL=3.28 (P=0.00066) at markers D5S2112-D5S820. These data and previous linkage findings from other investigators provide strong and consistent evidence for this genomic region as a susceptibility locus for schizophrenia. Exploratory analyses of a novel phenotype, psychosis, in families with schizophrenia and bipolar disorder detected evidence for linkage to the same markers as found in schizophrenia (peak NPL=3.03, P=0.0012 at D5S820), suggesting that this locus may be responsible for the psychotic symptoms observed in both diseases. Molecular Psychiatry (2004) 9, 213-218. doi:10.1038/sj.mp.4001418 Published online 30 December 2003

Phenotypic characterization and genealogical tracing in an Afrikaner schizophrenia database

Founder populations hold tremendous promise for mapping genes for complex traits, as they offer less genetic and environmental heterogeneity and greater potential for genealogical research. Not all founder populations are equally valuable, however. The Afrikaner population meets several criteria that make it an ideal population for mapping complex traits, including founding by a small number of initial founders that likely allowed for a relatively restricted set of mutations and a large current population size that allows identification of a sufficient number of cases. Here, we examine the potential to conduct genealogical research in this population and present initial results indicating that accurate genealogical tracing for up to 17 generations is feasible. We also examine the clinical similarities of schizophrenia cases diagnosed in South Africa and those diagnosed in other, heterogeneous populations, specifically the US. We find that, with regard to basic sample descriptors and cardinal symptoms of disease, the two populations are equivalent. It is, therefore, likely that results from our genetic study of schizophrenia will be applicable to other populations. Based on the results presented here, the history and current size of the population, as well as our previous analysis addressing the extent of background linkage disequilibrium (LD) in the Afrikaners, we conclude that the Afrikaner population is likely an appropriate founder population to map genes for schizophrenia using both linkage and LD approaches.

Genetic linkage in bipolar disorder

Bipolar disorder is an etiologically complex syndrome that is clearly heritable. Multiple genes, working singly or in concert, are likely to cause susceptibility to bipolar disorder. Bipolar disorder genetics has progressed rapidly in the last few decades. However, specific causal genetic mutations for bipolar disorder have not been identified. Both candidate gene studies and complete genome screens have been conducted. They have provided compelling evidence for several potential bipolar disorder susceptibility loci in several regions of the genome. The strongest evidence suggests that bipolar disorder susceptibility loci may lie in one or more genomic regions on chromosomes 18, 4, and 21. Other regions of interest, including those on chromosomes 5 and 8, are also under investigation. New approaches, such as the use of genetically isolated populations and the use of endophenotypes for bipolar disorder, hold promise for continued advancement in the search to identify specific bipolar disorder genes.

Recent advances in the genetics of schizophrenia

The high heritability of schizophrenia has stimulated much work aimed at identifying susceptibility genes using positional genetics. As a result, several strong and well-established linkages have emerged. Three of the best-supported regions are 6p24-22, 1q21-22 and 13q32-34 where single studies have achieved genome-wide significance at P<0.05 and suggestive positive findings have also been reported in other samples. Other promising regions include 8p21-22, 6q21-25, 22q11-12, 5q21-q33, 10p15-p11 and 1q42. Recently, evidence implicating individual genes within some of the linked regions has been reported and more importantly replicated. Currently, the weight of evidence supports NRG1 and DTNBP1 as schizophrenia susceptibility loci, though work remains before we understand precisely how genetic variation at each locus confers susceptibility and protection. The evidence for COMT, RGS4 and G72 is promising but not yet persuasive. While it is essential that further replications are established, the respective contributions of each gene, relationships with aspects of the phenotype, the possibility of epistatic interactions between genes and functional interactions between the gene products will all need investigation. The ability of positional genetics to implicate novel genes and pathways will open up new vistas for neurobiological research, and all the signs are that genetic research is poised to deliver crucial insights into the nature of schizophrenia.

Genomewide linkage scan for schizophrenia susceptibility loci among Ashkenazi Jewish families shows evidence of linkage on chromosome 10q22

Previous linkage studies in schizophrenia have been discouraging due to inconsistent findings and weak signals. Genetic heterogeneity has been cited as one of the primary culprits for such inconsistencies. We have performed a 10-cM autosomal genomewide linkage scan for schizophrenia susceptibility regions, using 29 multiplex families of Ashkenazi Jewish descent. Although there is no evidence that the rate of schizophrenia among the Ashkenazim differs from that in other populations, we have focused on this population in hopes of reducing genetic heterogeneity among families and increasing the detectable effects of any particular locus. We pursued both allele-sharing and parametric linkage analyses as implemented in Genehunter, version 2.0. Our strongest signal was achieved at chromosome 10q22.3 (D10S1686), with a nonparametric linkage score (NPL) of 3.35 (genomewide empirical P=.035) and a dominant heterogeneity LOD score (HLOD) of 3.14. Six other regions gave NPL scores >2.00 (on chromosomes 1p32.2, 4q34.3, 6p21.31, 7p15.2, 15q11.2, and 21q21.2). Upon follow-up with an additional 23 markers in the chromosome 10q region, our peak NPL score increased to 4.27 (D10S1774; empirical P=.00002), with a 95% confidence interval of 12.2 Mb for the location of the trait locus (D10S1677 to D10S1753). We find these results encouraging for the study of schizophrenia among Ashkenazi families and suggest further linkage and association studies in this chromosome 10q region.

Genome-wide scan in a large complex pedigree with predominantly male schizophrenics from the island of Kosrae: evidence for linkage to chromosome 2q

It is widely accepted that founder populations hold promise for mapping loci for complex traits. However, the outcome of these mapping efforts will most likely depend on the individual demographic characteristics and historical circumstances surrounding the founding of a given genetic isolate. The 'ideal' features of a founder population are currently unknown. The Micronesian islandic population of Kosrae, one of the four islands comprising the Federated States of Micronesia (FSM), was founded by a small number of settlers and went through a secondary genetic 'bottleneck' in the mid-19th century. The potential for reduced etiological (genetic and environmental) heterogeneity, as well as the opportunity to ascertain extended and statistically powerful pedigrees makes the Kosraen population attractive for mapping schizophrenia susceptibility genes. Our exhaustive case ascertainment from this islandic population identified 32 patients who met DSM-IV criteria for schizophrenia or schizoaffective disorder. Three of these were siblings in one nuclear family, and 27 were from a single large and complex schizophrenia kindred that includes a total of 251 individuals. One of the most startling findings in our ascertained sample was the great difference in male and female disease rates. A genome-wide scan provided initial suggestive evidence for linkage to markers on chromosomes 1, 2, 3, 7, 13, 15, 19, and X. Follow-up multipoint analyses gave additional support for a region on 2q37 that includes a schizophrenia locus previously identified in another small genetic isolate, with a well-established recent genealogical history and a small number of founders, located on the eastern border of Finland. In addition to providing further support for a schizophrenia susceptibility locus at 2q37, our results highlight the analytic challenges associated with extremely large and complex pedigrees, as well as the limitations associated with genetic studies of complex traits in small islandic populations.

Initial genome-wide scan for linkage with schizophrenia in the Japanese Schizophrenia Sib-Pair Linkage Group (JSSLG) families

To determine if there are common genes that contribute to the susceptibility for schizophrenia, first-stage genome-wide scan was carried out by genotyping 417 short-tandem repeat (STR) markers in 338 individuals from 130 families with 148 affected sib-pairs identified at 16 sites nationwide in Japan. Data was from the Japanese Schizophrenia Sib-pair Linkage Group (JSSLG), which is a multi-site collaborative study group established to create a national resource for genetic studies of schizophrenia in Japan. All subjects were Japanese, and the probands and their siblings had schizophrenia. Multipoint non-parametric linkage analysis and exclusion mapping were performed with GENEHUNTER software. Simulation studies suggested that in the absence of linkage we could expect one multipoint maximum LOD score (MLS) of 1.9 per genome scan. An MLS of 3.7 would be expected only once in every 20 genome scans and thus corresponds to a genome-wide significance of 0.05. No loci in the initial screen fulfilled the criteria for significant or suggestive evidence for linkage. Ten chromosomes (1, 2, 3, 4, 5, 8, 9, 14, 17, and 20) had at least one region with a nominal P value < 0.05. Susceptibility genes with lambdas of 3 and 2 were excluded from 98 and 70% of the genome, respectively. Our results suggest that common genes that contribute significantly to susceptibility for schizophrenia are unlikely to exist in the Japanese population.

Genome scan meta-analysis of schizophrenia and bipolar disorder, part II: Schizophrenia

Schizophrenia is a common disorder with high heritability and a 10-fold increase in risk to siblings of probands. Replication has been inconsistent for reports of significant genetic linkage. To assess evidence for linkage across studies, rank-based genome scan meta-analysis (GSMA) was applied to data from 20 schizophrenia genome scans. Each marker for each scan was assigned to 1 of 120 30-cM bins, with the bins ranked by linkage scores (1 = most significant) and the ranks averaged across studies (R(avg)) and then weighted for sample size (N(sqrt)[affected casess]). A permutation test was used to compute the probability of observing, by chance, each bin's average rank (P(AvgRnk)) or of observing it for a bin with the same place (first, second, etc.) in the order of average ranks in each permutation (P(ord)). The GSMA produced significant genomewide evidence for linkage on chromosome 2q (PAvgRnk<.000417). Two aggregate criteria for linkage were also met (clusters of nominally significant P values that did not occur in 1,000 replicates of the entire data set with no linkage present): 12 consecutive bins with both P(AvgRnk) and P(ord)<.05, including regions of chromosomes 5q, 3p, 11q, 6p, 1q, 22q, 8p, 20q, and 14p, and 19 consecutive bins with P(ord)<.05, additionally including regions of chromosomes 16q, 18q, 10p, 15q, 6q, and 17q. There is greater consistency of linkage results across studies than has been previously recognized. The results suggest that some or all of these regions contain loci that increase susceptibility to schizophrenia in diverse populations.

Association of neuregulin 1 with schizophrenia confirmed in a Scottish population

Recently, we identified neuregulin 1 (NRG1) as a susceptibility gene for schizophrenia in the Icelandic population, by a combined linkage and association approach. Here, we report the first study evaluating the relevance of NRG1 to schizophrenia in a population outside Iceland. Markers representing a core at-risk haplotype found in Icelanders at the 5' end of the NRG1 gene were genotyped in 609 unrelated Scottish patients and 618 unrelated Scottish control individuals. This haplotype consisted of five SNP markers and two microsatellites, which all appear to be in strong linkage disequilibrium. For the Scottish patients and control subjects, haplotype frequencies were estimated by maximum likelihood, using the expectation-maximization algorithm. The frequency of the seven-marker haplotype among the Scottish patients was significantly greater than that among the control subjects (10.2% vs. 5.9%, P=.00031). The estimated risk ratio was 1.8, which is in keeping with our report of unrelated Icelandic patients (2.1). Three of the seven markers in the haplotype gave single-point P values ranging from .000064 to .0021 for the allele contributing to the at-risk haplotype. This direct replication of haplotype association in a second population further implicates NRG1 as a factor that contributes to the etiology of schizophrenia.

Seasonal correlation of sporadic schizophrenia to Ixodes ticks and Lyme borreliosis

BACKGROUND: Being born in winter and spring is considered one of the most robust epidemiological risk factors for schizophrenia. The aetiology and exact timing of this birth excess, however, has remained elusive so far. Since during phylogeny, Borrelia DNA has led to multiple germ-line mutations within the CB1 candidate gene for schizophrenia, a meta analysis has been performed of all papers on schizophrenic birth excesses with no less than 3000 cases each. All published numerical data were then plotted against the seasonal distributions of Ixodes ticks worldwide. RESULTS: In the United States, Europe and Japan the birth excesses of those individuals who later in life develop schizophrenia mirror the seasonal distribution of Ixodes ticks nine months earlier at the time of conception. South of the Wallace Line, which limits the spread of Ixodes ticks and Borrelia burgdorferi into Australia, seasonal trends are less significant, and in Singapore, being non-endemic for Ixodes ticks and Lyme disease, schizophrenic birth excesses are absent. CONCLUSION: At present, it cannot be excluded that prenatal infection by B. burgdorferi is harmful to the implanting human blastocyst. The epidemiological clustering of sporadic schizophrenia by season and locality rather emphasises the risk to the unborn of developing a congenital, yet preventable brain disorder later in life.